1. Field
The present invention relates to electrical devices, and more specifically, to the circuitry in electrical devices for accommodating hot pluggable electrical devices.
2. Description of Related Art
Signal routing in high frequency systems can be challenging. Parasitic effects that can be ignored at lower frequencies become more pronounced at higher frequencies. Over the years circuit designers have developed various circuitry configurations in an effort to reduce parasitic effects. One engineering solution that reduces parasitic effects is the use of microstrip signal traces for circuitry and transmission lines. Microstrip signal traces are routed above (or below) a single reference layer such as a ground layer. The microstrip signal layer is often separated from the ground layer by a dielectric layer. Microstrip traces can be routed on either side of a given reference layer, or on both sides of the same reference layer. One drawback of microstrip is that the circuitry and transmission lines sometimes induce cross-talk, coupling noise into the signals. Microstrip can also be subject to higher levels of electromagnetic interference (EMI) as compared to other circuitry configurations designed to reduce parasitic effects such as stripline.
FIG. 1A depicts a stripline circuitry configuration, another popular configuration for mitigating parasitic effects. Stripline signal traces are routed between two reference layers, for example, between two ground layers or between a ground layer and a voltage reference layer (which may be called a power reference layer). In FIGS. 1A-B signal trace 101 is routed between reference layer 103 and ground layer 105. The voltage of reference layer 103 is connected to a voltage regulator module 107 to maintain a voltage of VDD. FIG. 1B is a circuit diagram corresponding to the stripline configuration of FIG. 1A. The stripline signal trace is typically separated from each of the two reference layers by insulative layers 109 and 111. The insulative layers may be dielectric substrate layers. Stripline tends to result in less noise coupling, better current return, more uniform field distribution, and better signal quality than microstrip. One drawback of it is that stripline circuits and transmission lines tend to have higher attenuation due to dielectric loss as compared to some other circuitry configurations.
The choice of whether to use a stripline signal trace with two ground layers or else a ground layer and a voltage layer sometimes depends upon the circuitry characteristics. Typically, with a push-pull driver, it is better practice to route stripline signals between power and ground layers, as shown in FIG. 1A, rather than using two ground layers. Using power and ground layers provides an efficient current return path. For example, an input/output (I/O) circuit using a push-pull type of driver has current return paths through either the power layer or the ground layer, depending on signal voltage transitions that are not harmoniously coupled with the ground-ground layer selection. An on-chip (or on-package) decoupling capacitor scheme can help mitigate the signal noise issues associated with current return path. But there is usually little or no available space on the chip to accommodate the amount of capacitance needed.
On the other hand, routing signals in a signal layer between a power layer and a ground layer is not always a viable solution due to the circuitry requirements of hot-pluggable devices. Hot plugging—sometimes called hot swapping—means that a component or peripheral of a device (e.g., keyboard of a computer or other peripheral device) can be plugged in, or removed and replaced, without powering the device down. Many of the latest keyboards, hard drives and other computer peripherals are hot pluggable and can be plugged in on the fly, without powering down the computer or rebooting it. Universal Serial Bus (USB) devices are hot swappable. While this is very convenient for users, hot pluggable components often suffer signal degradation of high frequency signals due to parasitic current effects in combination with the connector geometry. Hot-pluggable devices must have their own on-board voltage regulator module (VRM). The requirement of having multiple VRMs, and the current return path discontinuities caused by the connectors, often results in signal perturbations due to parasitic effects.
Today, hot-pluggable devices are becoming increasingly popular. The ability to swap out hot pluggable devices on the fly is useful in fixing system malfunctions without interrupting system operation. However, the conventional circuitry and board design practices for connecting hot pluggable devices create discontinuities in current return paths due to the use of multiple voltage regulator modules, printed circuit board complexities, and connector and decoupling capacitor geometries. These, in turn, cause signal degradation in conventional devices and hot pluggable devices. What is needed is a way to reduce the signal degradation of hot-pluggable devices that operate at high frequencies.